In the design for the multiple input/output system, the isolation between two antennas is closely associated to the correlation coefficient ρ in the communication system. Typically, while the smaller the correlation coefficient is, the higher the data throughput is. In general, the correlation coefficient ρ can be calculated based on the isolation between antennas and the matching through the following formula:
  ρ  =                                                                                      S                11                *                            ⁢                              S                12                                      +                                          S                21                *                            ⁢                              S                22                                                              2                              (                      1            -                                                                            S                  11                                                            2                        -                                                                            S                  21                                                            2                                )                ⁢                  (                      1            -                                                                            S                  22                                                            2                        -                                                                            S                  12                                                            2                                )                        
wherein S11 is the reflection coefficient of the first antenna, S22 is the reflection coefficient of the second antenna, and S12 and S21 are the degree of coupling between antennas. While the smaller S11 or S22 is, the better the matching of the first antenna or the second antenna is. It is known from the above formula that, in order to lower the correlation coefficient of a wireless communication system, there must be good matching of each antenna unit and low coupling between the two antennas, thus the data throughput of the wireless communication system is increased.
At present, the technique to lower the correlation coefficient is to add a complicated decoupling structure or circuit between the two antennas or on the two input terminals. However, in practical application the above techniques encounter the following drawbacks: 1. Decoupling structure will cause part of the energy to concentrate on the structure (a resonator for example), thus lowering the radiation efficiency of the antennas. 2. Decoupling structure has a certain size, thus occupying a relatively big area when applied to a handheld device, and the area of a handheld device is usually limited. 3. Decoupling circuit is composed of capacitors and inductors and the passive device and circuit must be designed for a specific frequency, thus increasing the complexity and cost. The above 3 drawbacks will make the practical application of MIMO antennas to handheld devices difficult to a certain extent and increase the cost. Therefore, we provide an innovative design utilizing a grounding metal piece to control the antenna resonance patterns (antenna mode, resonant frequency) in order to increase the isolation.
Accordingly, in view of the drawbacks of conventional technology, the Applicant, through careful testing and research and a spirit of perseverance, finally conceived the present invention “MULTIPLE-INPUT MULTIPLE-OUTPUT ANTENNA DEVICE”, which can overcome the above-mentioned drawbacks. The following is a concise description of the present application.